1. Field of the Invention
The present invention relates generally to a process for forming trenches in a semiconductor wafer and, more particularly, to a process for forming, with high precision, the trenches required to form various devices on a semiconductor wafer.
2. Description of the Related Art
In a typical semiconductor fabrication process, a plurality of trenches are formed in each semiconductor wafer, which are substrates of semiconductor devices. The pattern of the trenches is determined by the circuit design of the semiconductor devices to be produced. Such trenches, which are required to form, for example, separate transistor cells or capacitor cells, constitute an important constituent element which influences the basic characteristics of each device to be formed on the semiconductor wafer.
FIG. 1 is a top plan view showing the essential portion of a conventional semiconductor substrate wafer 10. A multiplicity of functional-pattern forming regions 1 which, in a subsequent step, will be cut into individual semiconductor chips are arranged over the single semiconductor substrate wafer 10 in orderly fashion. Each of the functional-pattern forming regions 1 has a trench forming section 2 which is provided for the purpose of forming the trenches required to form various kinds of devices and a marginal-pattern forming section 3 which is defined along the entire periphery of the trench forming section 2 so that electrodes and their associated elements may be formed thereon. The portions between adjacent functional-pattern forming regions 1 are called dicing lines 4. After various elements comprising a semiconductor chip have been formed in each of the functional-pattern forming regions 1, the semiconductor wafer 10 is cut into individual chips along the dicing lines 4.
As shown in FIGS. 2 and 3, trenches 5 are formed in the trench forming section 2. The trenches 5 constitute a pattern corresponding to individual devices which are to be formed in the trench forming section 2. Each of the trenches 5 has a cross-sectional shape such as that shown in FIG. 4.
Normally, such a trench 5 is formed by etching and it is necessary, therefore, to determine whether or not etching has reached a predetermined depth which represents the end of a trench-forming operation. A typical method of determining the end of a trench-forming operation is measurement of the fluctuations in the quantity of interfering light by using, for example, the apparatus shown in FIG. 5. In the illustrated apparatus, a semiconductor substrate wafer 10 is carried on a lower electrode 12 disposed in an etching chamber 11, and a through hole 13a is formed in an upper electrode 13 in the direction parallel to the optical axis of coherent light 18. The coherent light 18 which is emitted from a position above the etching chamber 11 irradiates a portion of the trench forming section 2 of the semiconductor substrate wafer 10 through the through hole 13a. The coherent light 18 is emitted from a light source 14 disposed at a position above the etching chamber 11 and is perpendicularly projected onto the semiconductor substrate wafer 10 via a spectroscopic prism 16 within a spectroscope 15.
Then, the coherent light 18 is reflected by the irradiated portion of a surface of the semiconductor substrate wafer 10. However, since the trenches 5 are formed in the surface portion of the semiconductor substrate wafer 10 as shown in FIG. 4, reflection of the coherent light 18 occurs on both a bottom surface 5b in the trench 5 and a surface 5a of an area between the trenches 5 (non-trenched surface). More specifically, as shown in FIG. 6, light 19a reflected from the non-trenched surface 5a of the semiconductor substrate wafer 10 and light 19b reflected from the bottom surface 5b in the trench 5 interfere with each other to form zero order diffracted light. The zero order diffracted light is made incident upon a light receiver 17 through the spectroscopic prism 16 in the spectroscope 15.
The phase difference between the reflected light rays 19a and 19b changes in accordance with the depth of the trench 5. It is possible, therefore, to detect the depth of the trench 5 by measuring the intensity of the zero order diffracted light with the light receiver 17. Also, as shown in FIG. 6, a resist 8 which serves as a mask for selective etching of the trenches 5 is formed over each of the non-trenched surfaces 5a.
The above-described conventional trench forming process, however, has the following problems. The trenches 5 formed in the semiconductor substrate wafer 10 constitute a planar pattern which corresponds to devices to be formed. Therefore, if the ratio of the total area of the non-trenched surfaces 5a to the total area of the bottom surfaces 5b of the trenches 5 within the area irradiated by the coherent light 18 is extremely large, the amplitude of the variation in the intensity of zero order diffracted light with respect to the depth of the trench 5 is limited to a narrow range as shown in FIG. 7. In FIG. 7, .lambda. indicates the wavelength of the coherent light 18.
In addition, each of the trenches 5 has a slightly different cross-sectional shape in accordance with its size. Accordingly, if the trenches 5 having various sizes exist within the area irradiated by the coherent light 18, the curve which represents variations in the intensity of the zero order diffracted light with respect to the depth of the trenches 5 becomes a distorted curve rather than a sinusoidal curve.
Thus, since the resolution and S/N ratio of measurement are degraded, it has been impossible to measure the depth of the trenches 5 with high precision, resulting in a reduction in the working accuracy in trench formation.